Aerogels are one of the lowest-density solid materials in existence and are made up of a nanoporous network of particles. Although their thermal conductivity can be lower than that of air, silica aerogels are very brittle, difficult to handle, and tend to absorb moisture. The latter compromises the strength indirectly by facilitating propagation of fractures. Therefore, practical use of these aerogels has been limited to specialized environments such as in nuclear reactors as the Cherencov radiation detectors or in space for collection of hypervelocity particles or for thermal insulation in space vehicles like the Sojourner Rover on Mars (1997). Although monolithic aerogels are desirable, currently, only aerogel beads are commercially available in large quantities, but beads suffer from severe settling problems due to their fragility. Monolithic aerogels are known in the art and can be prepared, for example, by various methods including the method disclosed in U.S. Pat. No. 4,432,956.
Aerogels are attractive materials for a variety of thermal insulation applications, however, this application has been slow because aerogels are fragile and difficult to handle. Therefore, it is desirable to encapsulate or coat aerogel monoliths with a harder skin or coating which does not compromise the bulk properties of the aerogel, but makes it easy to handle, transport and make into desirable products. The encapsulation may be in the form of a metal or ceramic coating. However, in cases where metals are undesirable due to corrosion or where the cost or difficulty in applying the ceramic coatings are prohibitive, a paint-on or spray-on coating of polymers precursors may be used. It is well known that aerogels collapse in contact with liquids, so that this method has not been attempted. In the present invention, we have found that we can coat aerogel monoliths by methods using viscous polymer precursors and subsequently curing the precursors into hard coatings without collapse of the monoliths. This is achieved by (a) controlling the amount of the coating precursor and/or (b) by curing the precursor coating to a hard layer before the monomeric or oligomeric precursor has time to percolate into the aerogel bulk. These principles have been demonstrated, for example, with isocyanates as the precursor which are cured by exposure to moisture in the environment to provide polyurethane/polyurea coatings. The same principles can be applied to other precursors such as the precursors of epoxies or polyimide precursors to obtain high temperature resistant protective layers or perfluorinated coatings for increased strength in combination with hydrophobicity or in combination with composite materials such as fibers to create aerogels encapsulated in a high strength shell.